1. Field of the Invention
The invention relates to a Q-switched semiconductor laser consisting of at least two electrically separated but optically coupled resonators of which one is structured as a passive resonator and the other one is structured as an active resonator.
Since short and intensive laser pulses can be generated with such arrangements, Q-switched semiconductor lasers are used, among others, in the field of non-linear optics and communication/optical data transmission.
In a Q-switched laser, a high population inversion is built up by strong pumping, and leads to a high amplification. But high resonator losses initially prevent their lasing activity. Once the losses are overcome, a short light pulse of high impulse intensity will be emitted. One way of accomplishing Q-switching is to alter the external losses at the resonator reflectors; another way is initially to produce internal losses in the resonator which losses may subsequently be eliminated.
2. The Prior Art
In IEEE Photonics Technology Letters, Vol. 7, No. 10, October 1995, pp. 1125-1127, there is described a dual-section DFB laser one section of which is initially operated as an absorber and which is thereafter made transparent by current injection. To this end, the net amplification in the resonator is modulated. Compared to laser modulation by pumping current, this arrangement yields improvements in respect of modulation currents and in the chirp. Yet high frequency laser modulation with low currents or voltage ranges have thus far not been realized.
U.S. Pat. No. 4,982,405 describes a Q-switched laser having two optically coupled resonators. As there described, Q-modulation is accomplished by detuning the two resonators.
The state of the art upon which the invention is based is described in IEEE Journal of Quantum Electronics 30 (1994), May, No. 5, pp. 1204-1211. The described Q-switched semiconductor laser consists of two optically coupled resonators one of which is structured as a passive resonator and the other of which is structured as an active resonator. Both resonators may be electrically switched (electrodes on the reflectors, common center electrode for both reflectors), whereby pumping of the active section (causing amplification) and tuning of the refractive index of the passive section may be carried out in separate operations. In this solution, too, the power modulation referred to as Q-modulation is accomplished by detuning of the two resonators. In this arrangement, the range having a variable refractive index, together with the two Bragg reflectors embracing this range, acts as a variable reflector or as a tunable etalon. DBR reflectors are used as resonator reflectors because of their high and substantially constant reflectivity within the stop band. In this arrangement, the DBR structure at the laser wavelength acts exclusively as a highly reflective mirror. While these solutions known from the state of the art permit high-frequency Q-modulation of the lasers, they do so at a low modulation range.